Sipcore R. Jesske
Internet-Draft Deutsche Telekom
Updates: RFC6442 (if approved) September 29, 2017
Intended status: Standards Track
Expires: April 2, 2018
ISUP Cause Location Parameter for the SIP Reason Header Fielddraft-ietf-sipcore-reason-q850-loc-01
Abstract
The SIP Reason header field is defined for carrying ISUP cause values
as well as SIP response codes. Some services in SIP networks may
need to know the ISUP location where the call was released in the
PSTN network to correctly interpret the reason of release.
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Internet-Draft ISUP Release Location Parameter September 20174. Mechanism
As defined by [RFC3326] a Reason header field MAY appear in any
request in a dialog, in any CANCEL request and in any response whose
status code explicitly allows the presence of this header field. The
syntax of the header field follows the standard SIP parameter syntax.
The mechanism employed adds a parameter with the ISUP location value
defined in [Q.850] to the Reason header field that identifies the
[Q.850] location of the call release in ISUP as defined in [Q.850] .
The location is a 4 bit value which reflects the possible locations
where an ISUP call is released. Some values are spare or reserved
for national use. The Augmented BNF (ABNF) [RFC5234] for this
parameter is shown in Figure 1.
reason-extension =/ isup-cause-location
isup-cause-location = "location" EQUAL string
The following values shall be used as location:
U for 0 0 0 0 user
LPN for 0 0 0 1 private network serving the local user
LN for 0 0 1 0 public network serving the local user
TN for 0 0 1 1 transit network
RLN for 0 1 0 0 public network serving the remote user
RPN for 0 1 0 1 private network serving the remote user
LOC-6 for 0 1 1 0 spare
INTL for 0 1 1 1 international network
LOC-8 for 1 0 0 0 spare
LOC-9 for 1 0 0 1 spare
BI for 1 0 1 0 network beyond interworking point
LOC-11 for 1 0 1 1 spare
LOC-12 for 1 1 0 0 reserved for national use
LOC-13 for 1 1 0 1 reserved for national use
LOC-14 for 1 1 1 0 reserved for national use
LOC-15 for 1 1 1 1 reserved for national use
Figure 1: isup-cause-location
Note: These are the values defined within [Q.850] as location. Thus
other values are not within the scope of this document.
Depending on the direction the UAC or UAS shall include the isup-
cause-location when setting up the Reason header field with a [Q.850]
cause. This approach is only valid in cases when the ISUP [Q.850]
location is available.
Jesske Expires April 2, 2018 [Page 3]

Internet-Draft ISUP Release Location Parameter September 20175. Example
The following example shows a SIP 404 response message containing a
Reason header field with a [Q.850] cause value and a isup-cause-
location value. The 404 Response will be set up when a gateway
receives an ISUP Release with a [Q.850] cause set to 1 meaning
"Unallocated (unassigned) number", i.e. the number is not known in
the PSTN.
404 Not Found
SIP/2.0 404 Not Found
From: Alice <sips:alice@atlanta.example.com>;tag=1234567
To: Bob <sips:bob@biloxi.example.com>;tag=765432
Call-ID: 12345600@atlanta.example.com
CSeq: 1 INVITE
Reason: Q.850;cause=1;text="Unallocated (unassigned) number";
location=LN
Content-Length: 0
Figure 2: Example Location in Reason header field.
6. Privacy Considerations
This document doesn't change any of the privacy considerations
described in [RFC3326]. While the addition of the isup-cause-
location parameter does provide an indicator of the entity that added
the location in the signaling path this provides little more exposure
than the [Q.850] cause itself.
7. Security Considerations
This document doesn't change any of the security considerations
described in [RFC3326]. The addition of the isup-cause-location
parameter does provide an indicator of the [Q.850] location where the
call was released within the PSTN. This information may be used for
specific location driven services but does not create any additional
security constrains. But since the [Q.850] location is very
imprecise the [Q.850] location value itself will not add any major
security constrain. The use of this parameter is not restricted to a
specific architecture.
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